Move the Flycheck menu under Tools.

Minor modes are not supposed to define top-level
menus in Emacs. The proper thing for them is generally to
add a menu item under Tools. This particular commit
positions the Flycheck menu item just above "Spell Checking"
and renames it to "Syntax Checking"
Merge pull request #59 from bbatsov/fix-menu

Move the Flycheck menu under Tools.
Fix error patterns of CSSLint checker

Properly differentiate between errors and warnings.
Improve checker names

Append the name of the tool to the name of the checker unless the
checker tool is the authoritative compiler/checker for the language in
question.
[#59] Fix menu setup

`easy-menu-add-item` is not autoloaded, and apparently not considered as
part of the public interface.  Use `easy-menu-change` instead which is
nicer anyway.